1. Field of the Invention
The present invention relates to a monopole antenna mainly used for mobile communication, and more specifically, to a monopole antenna suitable for base stations.
2. Description of the Related Art
Prior art antennas are shown in FIGS. 36 and 37.
A first prior art antenna shown in FIG. 36 will be described as follows. FIG. 36 shows one technique to change the directivity of the vertical plane of the antenna, and FIGS. 37(A)-37(D) show examples of radiation directivity of monopole antennas.
FIG. 36 shows a ground conductor 111, a coaxial power supply part 112, and an antenna element 113 connected to the coaxial power supply part 112 on the ground conductor 111. In this case, a monopole antenna has an axis symmetric structure where the ground conductor 111 is shaped like a disk, the coaxial power supply part 12 is located at the center of the surface of the ground conductor 111, and the antenna element 113 is connected to the coaxial power supply part 12 in a manner to be perpendicular to the ground conductor 111. The radio waves of the antenna are nondirectional with respect to the horizontal plane of the antenna.
A method of changing the directivity of the radio waves on the vertical surface in a monopole antenna is to change the size of the ground conductor 111. When the ground conductor 111 have a finite size, radio waves diffract at the edge of the ground conductor 111. The size of the diffraction depends on the size of the ground conductor 111; the larger the ground conductor 111 is, the smaller the diffraction becomes, and vice versa. The entire radio waves of the antenna are the sum of the radio waves from the antenna element 113 and the diffraction waves from the edge of the ground conductor 111. If the antenna is divided into two sides: the top side having the antenna element 113 and the bottom side below the ground conductor 111, fewer radio waves flow to the bottom side and more radio waves are applied to the top side with increasing the ground conductor 111 in size. Also, the maximum radiation direction approaches the horizontal plane of the antenna. On the other hand, as the ground conductor 111 becomes smaller, more radio waves flow to the bottom side, making the maximum radiation direction approach the upright direction of the antenna. However, when the diameter of the ground conductor 111 is equal to or below xc2xd wavelength, the radio waves flow equally to the top and bottom sides, exhibiting directivity in the form of the number 8 on the vertical plane of the antenna. At this moment, the maximum radiation direction is the horizontal plane of the antenna. FIGS. 37 show the radiation directivity when the ground conductor 111 has a diameter of about xc2xd wavelength (37A), about 0.8 wavelength (37B), and about 3wavelength (37C). In FIGS. 36 and 37, X and Y indicate the direction parallel to the surface of the ground conductor 111 and Z indicates the direction perpendicular to the ground conductor 111. The radio directivity is calibrated in 10 dB, and the unit used is dBd, referred to the gain of a dipole antenna.
Thus a monopole antenna can change the directivity of the radio waves on the vertical plane of the antenna by changing the ground conductor 111 in size.
The second prior art antenna will be described with reference to FIG. 38 showing a technique to change the directivity of an antenna. FIG. 38 illustrates a monopole antenna array provided with two antenna elements, and FIG. 39 shows an example of radiation directivity.
The antenna array comprises a ground conductor 121, coaxial power supply parts 122 and 123, antenna elements 124 and 125, power supply paths 126 and 127, and a power distribution/composition circuit 128. The antenna elements 124 and 125 are connected to the coaxial power supply parts 122 and 123, respectively, on the ground conductor 121. The coaxial power supply parts 122 and 123 are connected to the power distribution/composition circuit 128 via the power supply paths 126 and 127, respectively. The ground conductor 121 is provided on the XY plane.
The following will describe the case where there are two antenna elements 124 and 125, and radio waves are strong in the X axis direction.
The antenna elements 124 and 125 are arranged xc2xd wavelength apart from each other on the X axis to be symmetric with respect to the origin point, and currents to be supplied have a phase difference of 180 degrees. At this moment, the array factors become co-phase in the +X and xe2x88x92X directions to reinforce each other. When the antenna is symmetric with respect to the ZX plane and the ZY plane, the radio waves become symmetric with respect to the ZX plane and the ZY plane. The waves to be radiated become strong in the +X direction and the xe2x88x92X direction where the radiation waves from the antenna elements 124 and 125 have the same phase. Furthermore, changing the size of the ground conductor 121 or the distance between the antenna elements allows the directivity of the radio waves on the vertical plane of the antenna to change.
FIG. 39 shows as an example the radiation directivity when the antenna elements are made of a xc2xc wavelength metallic wire, the antenna elements are supplied with power at a one to one ratio, and the ground conductor is a rectangle having one side of 2.75 wavelength parallel to the X axis and the other side of 2.25 wavelength parallel to the Y axis. In FIG. 39, X and Y. indicate the direction parallel to the plane of the ground conductor 121, and Z indicates the direction perpendicular to the ground conductor 121. The radio directivity is calibrated in 10 dB, and the unit is dBd, referred to the gain of a dipole antenna.
Thus, an antenna capable of changing the directivity of radio waves is achieved by arranging the antenna elements so as to form an array at an appropriate interval and by providing the antenna elements with an appropriate phase difference and an appropriate power distribution ratio.
However, the first prior art antenna has the following drawback; intensifying the radiation in the horizontal direction of the antenna requires a two-dimensionally large ground conductor 111, which is against miniaturization of the monopole antenna. A monopole antenna is not allowed to occupy so large an area on the ceiling, which is one of the best sites indoors for a monopole antenna. Hence the first prior art antenna, which must be large in size because of its being difficult to be small two dimensionally, is unsuitable.
On the other hand, the second prior art antenna can intensify radio waves by providing directivity in the horizontal direction of the antenna. However, it requires to have the power supply paths 126 and 127 and the power distribution/composition circuit 128, which causes a intrinsic loss in these components 126, 127, and 128 due to the structure of the circuit. Another loss is caused when the waves radiated from one antenna element 124 (125) are undesirably received by the other antenna element 125 (124) due to poor isolation between the antenna elements. These losses deteriorate the radiation efficiency. The latter-mentioned loss in particular leads to a reflection loss as the entire antenna array, and the reflected signal may reversely flow to each device connected to the antenna, thereby badly affecting the characteristics of each device. In order to secure excellent antenna characteristics, the former-mentioned loss should be reduced in the power supply paths and the power distribution/composition circuit 128, and the latter case requires to establish good isolation between the antenna elements. In the former case, components having a fewer loss can be employed as the power supply paths 126 and 127 and the power distribution/composition circuit 128. The latter case needs to extend the distance between the antenna elements. Hence, the antenna array in the second prior art is unsuitable for miniaturization of an antenna. When there are more than two antenna elements, the distance between them is considered to become larger than in the second prior art antenna which have two antenna elements. The large-scale antenna array is unsuitable for miniaturization of an antenna. A monopole antenna is not allowed to occupy so large an area on the ceiling, which is one of the best sites indoors for a monopole antenna. Hence the second prior art antenna, which must be large in size because of its being difficult to be small two dimensionally, is also unsuitable.
When an antenna is installed on a ceiling, in order to enhance the efficiency of wave radiation, it is preferable to hang the antenna elements upside down from the ceiling so as to make them face the space into which radio waves are radiated. It is further preferred that there is nothing to disturb the propagation of the radio waves between the antenna and the entire radiation space, and that the space including the entire radiation objects can be seen from the antenna elements. It is further desired to install a monopole antenna inconspicuously not to be an eyesore; however, in the prior art antennas shown in FIGS. 36 through 39 the antenna elements project from the ceiling unsightly, and the first and second prior art antennas cannot satisfy the demand due to their failure to be miniaturized.
In view of the above problems, the main object of the present invention is to provide an antenna, which is small in size, particularly its top side, and capable of changing the directivity of radio wave.
In order to achieve the object, the present invention comprises a ground conductor; a power supply part arranged on a surface of said ground conductor; an antenna element connected to said power supply part; and a side conductor surrounding a space including said antenna element apart from said antenna element. Consequently, the wave radiation along the horizontal plane of the antenna can be intensified without increasing the two-dimensional size very much. The reason for this is as follows. The side conductor functions as the periphery of the ground conductor to prevent wave diffraction effectively, thereby intensify wave radiation in the horizontal direction of the antenna. Furthermore, the side conductor is arranged above the ground conductor, which hardly increases the two-dimensional size of the antenna.
It is preferred that the antenna is further provided with a ceiling conductor facing said ground conductor with said antenna element therebetween because this structure can reduce the size of the antenna in the vertical direction. Since the ceiling conductor functions as the tip of the antenna element, the antenna element can be reduced in length by that. The antenna is reduced in size in the vertical direction accordingly.
It is preferred that the edge portion of said ceiling conductor is electrically connected to said side conductor, because this structure allows the directivity of the radio waves along the horizontal plane to be adjusted as desired. The reason for this is as follows. When the edge portion of the ceiling conductor is connected to the side conductor, current leaks form there towards the ground conductor. As a result, radio waves are hardly radiated in the direction extending outside from the ceiling conductor along the connection point. Setting the direction along which the connection point of the ceiling conductor and the side conductor is provided can set the directivity of the waves along the horizontal plane.
It is preferred that said ceiling conductor has a circular central portion because this allows the directivity of the waves along the horizontal plane to be adjusted more freely. The reason for this is as follows. When the edge of the ceiling conductor is connected to the side conductor, the minimum point of the waves is formed in the direction extending outside along the connection point, which enables the adjustment of the directivity. However, there are cases where the radiation level is lower than desired at the minimum point. In contrast, the circular center of the ceiling conductor allows waves to be radiated from the entire circumference of the circular portion, making the wave radiation approximately nondirectional on the horizontal plane. As a result, the radiation of waves becomes a mixture of the radiation from the circular portion and the radiation from the remaining portion, thereby compensating the minimum point of the waves. The amount of radiation of waves from the circular portion can be adjusted by changing the size of the circular portion.
It is preferred that said side conductor is electrically connected to said ground conductor, because it can achieve the matching of input impedances. The reason for this is as follows. When the antenna is reduced in size in the vertical direction by providing the ceiling conductor, the ceiling conductor and the ground conductor are arranged close to each other, which causes capacitive component between them, leading to mismatching of the input impedances. In contrast, electrically connecting the ceiling conductor to the ground conductor via the side conductor causes a continuity loop between these conductors, which develops an inductance. Consequently, the inductance compensates the capacitive component, thereby solving the mismatching of the impedances.
It is preferred that at least one of said ground conductor, said side conductor, and said ceiling conductor has an opening because the set wave directivity can be set as desired by adjusting the position, size, and other conditions of the opening when it is formed.
It is preferred that the antenna is provided with means for adjusting the size of said opening. Adjusting the size of the opening allows the directivity and impedances to be fine adjusted after the formation of the opening.
It is preferred that said power supply part is arranged on the origin point, said ground conductor is arranged on the XY plane, said ground conductor and said side conductor are designed to be symmetric with respect to the ZY plane, and said opening is arranged to be symmetric with respect to the ZY plane. This structure allows the directivity of waves to be symmetric with respect to the ZY plane.
It is preferred that said ground conductor and said side conductor are designed to be symmetric with respect to the ZX plane, and said opening is arranged to be symmetric with respect to the ZX plane. This structure allows the directivity of waves to be symmetric with respect to the ZX plane.
It is preferred that said antenna element is electrically connected to said ceiling conductor. This stabilizes not only the structure of the antenna but also the impedance of the antenna, thereby improving the characteristics of the antenna.
It is preferred that the antenna is provided with a dielectric member having permittivity higher than air in a space surrounded by said ground conductor and said side conductor. This makes the antenna compact and low-profile.
It is preferred that said space is filled with said dielectric member. This makes the antenna compact and low-profile. In addition, there is no clearance inside the antenna that brings dust inside the space or causes condensation, thereby improving the reliability.
It is preferred that said dielectric member is structured as a lid for the space surrounded by said side conductor, and either,said ground conductor or said ceiling conductor is provided on said dielectric member. This develops no clearance inside the antenna that brings dust inside or causes condensation, thereby improving the reliability. Furthermore, the space inside the antenna can be easily sealed by using the dielectric member as a lid.
It is preferred that said side conductor is made of a via hole formed in said dielectric member. This facilitates the formation of the side conductor because the via hole can be formed comparatively easily by a general substrate production method.
It is preferred that the antenna is provided with at least one matching element arranged apart from said antenna element, said matching element being electrically connected to said ground conductor. This changes the impedance of the antenna to establish good matching conditions.
It is preferred that at least one said matching element is electrically connected to said antenna element. This increases the input impedance of the monopole antenna.
It is preferred that at least one said matching element is electrically connected to said ceiling conductor. This changes the input impedance of the monopole antenna.
It is preferred that a radio device comprising: a monopole antenna of the present invention; amplification means for amplifying transmission signals supplied to said monopole antenna and reception signals supplied from said monopole antenna; frequency selection means for selecting frequencies of the transmission signals and reception signals; and a cabinet for storing said monopole antenna and said amplification means, and is also preferred that said cabinet is provided with a concave portion on a surface thereof for storing said monopole antenna inside. Consequently, a radio device far from being an eyesore can be achieved while maintaining or improving the compact and low-profile characteristics. This is because the monopole antenna is stored in the concave portion on the cabinet surface, making the antenna hard to be seen from outside. Since the compact and low-profile characteristics of the antenna are improved, incorporating the antenna inside the device does. not disturb the compact and low-profile characteristics of the radio device.
It is preferred that an arrangement structure of a monopole antenna comprises a plurality of monopole antennas of the present invention arranged in a manner to conform the direction for minimizing the directivity of the horizontal plane of each of said monopole antennas. As a result, the wave transmission of adjacent monopole antennas has minimum influence to each other, thereby establishing excellent isolation between them.